Some simple batteries are made up of a single cell having a single voltage. For example, a common A cell battery is made up of a single 1.5, volt cell. However, many other batteries are made up of multiple battery cells that are connected together in series. For example, a common car battery could be made up of six 2, volt cells connected in series, while a battery for a hybrid car might be made up of an array of battery cells, some in series and some in parallel. Likewise, an electronic device might employ a battery made of a chain of smaller batteries connected in series.
In multiple-cell batteries, small differences between the cells (e.g., due to production tolerances or operating conditions) tend to be magnified with each charge or discharge cycle. In these situations, weaker cells are overstressed during charging, causing them to become even weaker, until they eventually fail and cause a premature failure of the whole battery. Voltage equalization (sometimes called cell balancing) is a way of compensating for these weaker cells by equalizing the charge on all the cells in the battery, thus extending the battery's life. It also serves to prevent the overcharge of any individual cell.
Even in a relatively stable battery, small differences in charge, rate of discharge, cell parameters, and cell age can result in different cell voltages for individual cells after a few charge and discharge cycles, leading to undesirable stress on the battery as the individual cell voltages begin to slide out of synchronization.
Therefore, in order to obtain good performance and lifespan for most rechargeable batteries, such as Li-ion batteries, each battery cell requires additional circuitry. The level of sophistication of these circuits varies widely and so does the cost. The simplest and lowest cost devices typically provide only passive equalization. In such a device, should the voltage of an individual cell exceed a preset level, a simple discharge circuit attached to the cell is activated to bring the cell voltage back to what is considered a “normal level.” In such a device, any “excess” charge is wasted, typically turned into heat.
It would be desirable, therefore to provide a system and method of actively equalizing charge among cells in a multiple-cell battery. Such a system could periodically monitor the voltage in each battery cell in a battery or battery portion, and equalize the voltages in the batteries so that they all remain at the same voltage, or at least close to the same voltage. Furthermore, it would also be desirable if any charge removed from an overcharged battery were not wasted, but was instead moved to an undercharged battery instead, avoiding any wasted charge.
However, there is also a competing interest to keep the size, cost, and complexity of batteries low. Therefore, it would also be desirable for any active battery equalization system to be simple, energy efficient, small, and low cost.